The present invention relates to a process for preparing 2-methylbutanal from secondary streams obtained in the preparation of mixtures of isomeric α,β-unsaturated decenals. Pentanals, also called valeraldehydes, have gained economic significance as intermediates in industrial organic chemistry. They occur in four different structural isomers as linear n-pentanal, branched 2-methylbutanal, branched 3-methylbutanal and highly branched 2,2-dimethylpropanal or pivalaldehyde. Pentanals can be used as such, for example for the production of fragrances, or in the form of the derivatization products thereof, such as pentanols, pentanoic acids or pentylamines. The pentanals can be processed in pure isomeric form or in the form of an isomer mixture (Weissermel, Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim, 1988, page 218; Schneidmeir, Chemiker-Zeitung, volume 96 (1972), no. 7, pages 383-387).
Because of the reactive aldehyde group, pentanals in a basic medium can enter into an aldol addition reaction to give pentanal dimerization products having ten carbon atoms. If at least one of the pentanal isomers has two reactive hydrogen atoms in the α position to the carbonyl group, the aldol addition product formed at first can be converted to the α,β-unsaturated aldol condensation product or α,β-unsaturated decenal with elimination of water. The formation of the double bond in the α,β position relative to the carbonyl carbon atom with elimination of water is frequently also referred to as crotonization.
In the aldol condensation of pentanals, either structurally identical pentanals can react in a self-condensation or structurally different pentanals in a co-condensation. Among the pentanals, linear n-pentanal has the greatest reactivity, and the aldol condensation of n-pentanal-containing pentanal mixtures preferentially affords 2-propylheptenal from the self-condensation of n-pentanal. The aldol addition reaction and aldol condensation reaction of pentanals is typically conducted in the presence of basic catalysts such as aqueous alkali metal hydroxide solutions.
The aldol condensation reaction of pentanals to give mixtures of isomeric α,β-unsaturated decenals can be conducted, for example, in a tubular reactor as described in DE 10 2009 001594 A1 and DE 199 57 522 A1, in a stirred tank as known from EP 0 366 089 A2, or in a reaction mixing pump according to DE 10 2009 045 139 A1. According to the proportions of the isomeric pentanals in the feed mixture, mixtures comprising structurally different α,β-unsaturated decenals are obtained.
U.S. Pat. No. 4,426,542 A likewise concerns the aldolization of a pentanal mixture in the presence of an aqueous alkali metal hydroxide solution and subsequent hydrogenation to give a mixture of isomeric decanols. Unreacted aldehydes can be removed by distillation from the decenal mixture prior to the hydrogenation stage. The isomeric pentanals are obtainable by hydroformylation reaction of linear butenes.
The resultant mixtures of isomeric α,β-unsaturated decenals likewise have increasing economic significance as organic intermediates. The complete hydrogenation of the olefinic double bond and the aldehyde group affords a mixture of structurally isomeric decanols which are processed further by esterification with aromatic di- or polycarboxylic acids and aliphatic dicarboxylic acids to give plasticizers for thermoplastics. The dependence of the plasticizer properties on the composition of the decanol mixture is examined, for example, in EP 0 366 089 A2. The use of isomeric decanol mixtures as plasticizer alcohols which are obtained by complete hydrogenation of mixtures of isomeric α,β-unsaturated decenals is likewise described in DE 42 100 26 A1 and DE 43 33 324 A1.
α,β-Unsaturated decenals can likewise be hydrogenated selectively to give a mixture of isomeric decanals which, because of their aldehyde reactivity, can be converted to further conversion products, especially to carboxylic acids by oxidation (DE 10 2009 027 978 A1). Isomeric decanoic acids find use for production of lubricant esters or serve for preparation of peracids for polymerization reactions.
Pentanals are prepared industrially by reaction of butenes with synthesis gas, a mixture of carbon monoxide and hydrogen, in the presence of transition metal compounds. The reaction of olefins with synthesis gas is also referred to as the hydroformylation reaction or oxo reaction, and the hydroformylation of but-1-ene affords, as well as the straight-chain n-aldehyde n-pentanal, also certain proportions of the isoaldehyde 2-methylbutanal (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, vol. 2, pages 73-74; vol. 25, pages 286-289).
Butenes are obtained industrially by the steamcracking of naphtha. Since the linear butenes but-1-ene and but-2-ene are of significance for the preparation of mixtures of isomeric α,β-unsaturated decenals, it is customary to first separate 1,3-butadiene from the butene cut from the naphtha cracking to form raffinate I, and then to remove isobutene to form raffinate II (Weissermel, Arpe, Industrielle Organische Chemie, 3rd edition, VCH Verlagsgesellschaft mbH, 1988, pages 71-79). For the subsequent hydroformylation reaction, predominantly raffinate II is used, in which a small residual isobutene content can be permitted. In special cases, it is also possible to process raffinate I having a high isobutene content, and occasionally also a but-1-ene-depleted raffinate II, which can also be referred to as raffinate III.
The hydroformylation reaction can be conducted either in the presence or in the absence of complex-forming compounds, for example in the presence of organophosphorus compounds. According to EP 0 366 089 A2, a homogeneous organic solution is employed with rhodium triphenylphosphine catalysis. Since the aim is generally a maximum proportion of n-pentanal compared to 2-methylbutanal in the pentanal mixture formed, the hydroformylation reaction is frequently conducted in the presence of homogeneously dissolved transition metal complexes, which first enable isomerization of the but-2-ene to but-1-ene, which is then hydroformylated predominantly to n-pentanal. Rhodium complex catalysts suitable for the isomerizing hydroformylation of a mixture of linear butenes are described, for example, in DE 102 25 282 A1, in which the complex ligands have a xanthene skeleton.
Rhodium complex catalysts based on bisphosphite ligands together with sterically hindered secondary amines, which are likewise suitable for the isomerizing hydroformylation of a mixture of linear butenes, are discussed in DE 10 2008 002 187 A1. Two-stage process variants are also known, for example according to DE 43 33 324 A1, DE 42 10 026 A1, DE 101 08 474 A1 and DE 101 08 475. In the first stage, but-1-ene is preferentially converted, while, in the second stage, the but-2-ene-containing offgas from the first stage is hydroformylated to give a mixture of n-pentanal and 2-methyl-butanal. According to DE 43 33 324 A1 and DE 101 08 474 A1, the first hydroformylation stage can also be conducted in the presence of water-soluble rhodium complex catalysts. In this type of reaction regime, a liquid aqueous catalyst solution is present alongside the liquid organic reaction solution, which, after leaving the hydroformylation zone, can be separated from one another in a simple manner by phase separation. Because of the presence of an aqueous phase and an organic liquid phase, this type of reaction regime is also referred to as a heterogeneous process or biphasic process.
Because of the increasing demand for mixtures of isomeric α,β-unsaturated decenals, the hydroformylation reaction of the butene feed mixture gives considerable amounts of 2-methyl-butanal and to a certain degree also 3-methylbutanal, even when the use of suitable catalysts which promote isomerizing hydroformylation results in preferential n-pentanal formation. There is therefore a need for a process in which the 2-methyl-butanal obtained in the preparation of mixtures of isomeric α,β-unsaturated decenals can be obtained in maximum quality and free of impurities, such that the 2-methylbutanal obtained can be used with maximum versatility and the overall process for production of mixtures of isomeric α,β-unsaturated decenals can thus be made more economically viable.